[straitlineguy]

I'm trying to install a bathroom off my garage. I have a 4" pvc tee'd into the main drain and want to have three 2" drains tied into it. The roof vent is one foot downgrade from the toilet, and then the shower and sink will be four and eight feet upgrade from that. Do I have to run a vent from the furthest sink drain up into the attic and then tie back into that roof vent? I don't see why I would need to, but my nieghbor seems to think if I don't, the sink drain will run slow. I can't wrap my head around how a half inch water supply line can fill a four inch drain to cause it to drain slow. What am I missing? Thanks in advance for your advice.

 

[Hammertime]

I'm trying to install a bathroom off my garage. I have a 4" pvc tee'd into the main drain and want to have three 2" drains tied into it. The roof vent is one foot downgrade from the toilet, and then the shower and sink will be four and eight feet upgrade from that. Do I have to run a vent from the furthest sink drain up into the attic and then tie back into that roof vent? I don't see why I would need to, but my nieghbor seems to think if I don't, the sink drain will run slow. I can't wrap my head around how a half inch water supply line can fill a four inch drain to cause it to drain slow. What am I missing? Thanks in advance for your advice.

Your neighbor is right. It's not the draining of the water for the 4" pipe. It's the draining of the water into the 2" pipe. A vent has to be within so many feet of the sink, toilet, and shower. Call your local plumbing inspector because you won't pass an inspection without it.
The best way I can think to explain it is open a can of punch or juice. Open one end and try pouring it. It will gurgle out because the air has no where to go. Now open the other end. It pours out much smoother and faster. Don't get me wrong. It will still drain. But, not by professional standards nor code enforcement standards.

 

[PipeGuy]

Do I have to run a vent from the furthest sink drain up into the attic and then tie back into that roof vent?

Some jurisdictions will allow the use of a vacuum valve, in lieu of a vent, for certain drain fixtures. It's an inexpensive device that lets air into the drain line, to facilitate proper drainage, but doesn't let sewer gas out. Again, check with your local code enforcement office for specifics that apply to you.